The present invention relates to an extrusion die for extruding a honeycomb structure, especially to an extrusion die for extruding a honeycomb structure having a thick outer peripheral wall.
The honeycomb structure for use as a catalyst support for purifying exhaust gases of a vehicle, or the like is composed of a large number of grid-shaped thin walls, and a large number of open passages separated by the thin walls.
The honeycomb structure having such a construction as described above is made of a fragile material such as ceramic so that the mechanical strength thereof becomes low.
In order to improve the mechanical strength, a thick outer peripheral wall is integrally formed on the peripheral surface of the honeycomb structure.
The conventional extrusion die for forming a honeycomb structure provided with such a thick outer peripheral wall as described above, is composed of a die body 1 and a die mask 2 which is disposed so as to be opposed to the outlet face of the die body 1. And between the outlet face of the die body 1 and the opposed end surface of the die mask 2, an annular opening portion 3 is formed as shown in FIGS. 1, 2 and 3.
The honeycomb structure provided with a thick outer peripheral wall is conventionally produced by the following steps.
(1) At first, the material to be extruded, is forcedly supplied into feed passageways 4 which are disconnectedly formed on the inlet side of the die body 1.
(2) Then, the material flows into interconnected grid-shaped extrusion slots 5 having the cross sectional shape conforming to that of the honeycomb structure, which are formed on the outlet side of the die body 1 so as to be communicated with the feed passageways 4 within the die body 1, and the material moves straightly within the extrusion slots while being laterally spread therewithin so as to become a coherent grid-shaped body.
(3) Finally, the material straightly moving through the extrusion slots 4 situated in the central portion of the die body 1 is extruded out of the die body 1 into the inside of the die mask 2 and is formed into a honeycomb structure, and the material extruded into the opening portion 3 inwardly moves to crush open passages formed in the outer peripheral portion of the honeycomb structure which is extruded out of the central extrusion slots to integrally form a thick outer peripheral wall about the honeycomb structure.
However, since the peripheral portion of the honeycomb structure which is extruded out of the central extrusion slots 5 is crushed by the material extruded into the opening portion 3, distortion is produced in the outer peripheral portion of the obtained honeycomb structure in the final step (3) as described above.
Therefore, when the obtained honeycomb structure is dried and fired and when the obtained product is used, cracks are apt to be generated in the outer peripheral portion thereof.
And when the honeycomb structure provided with very thin walls is extruded, the thin walls adjacent to the thick outer wall are apt to be deformed due to the pressure applied by the material extruded into the opening portion 3 in the final step (3) as described above.
Accordingly, one object of the present invention is to provide an extrusion die for extruding a honeycomb structure provided with a thick outer peripheral wall and having excellent mechanical strength.
Another object of the present invention is to provide an extrusion die by which a thick outer peripheral wall can be integrally formed about the outer peripheral portion of a honeycomb structure without generating any distortion therein.
Still another object of the present invention is to provide an extrusion die suitable for extruding a catalyst support for purifying exhaust gases, which is composed of a large number of narrow open passages separated by thin partition walls, and a thick outer peripheral wall integrally formed on the outer periphery thereof, and which has excellent strength.